Configure an Unnumbered Interface

When you need to conserve IP addresses, you can configure unnumbered interfaces.
To do this, configure the protocol family, but do not include the address statement at the [edit interfaces interface-name unit logical-unit-number family family] hierarchy level:

[edit interfaces interface-name unit logical-unit-number] 

family family; 

For example:

[edit]

interfaces {

    so-6/1/0 {

        unit 0 {

            family inet;

            family iso;

        }

    }

}

When configuring unnumbered interfaces, you must ensure that a source address is configured on some interface in the router. This address is the default address. We recommend that you do this by assigning an address to the loopback interface (lo0), as described in Configure the Loopback Interface. If you configure an address (other than a martian) on the lo0 interface, that address is always the default address, which is preferable because the loopback interface is independent of any physical interfaces and therefore is always accessible.